1. Field of the Invention
The present invention relates to a technique for controlling a printing device wherein ink droplets with a plurality of dot diameters are recorded.
2. Description of the Related Art
Some of recent ink jet printers are so constituted that ink can be discharged through their nozzles with various dot diameters, large, medium, and small. In such printers, the gradation of each pixel can be represented in two or more levels according to the presence or absence of these dots. In this type of ink jet printer, image data is converted into dot data by one-dimensional tables, common to every color, wherein color component values are in correspondence with the recording rates of large, medium, and small dots. The image data is composed of color component values, and the dot data is composed of the recording rates of large, medium, and small dots. Then, each dot data is subjected to half tone processing or the like, and print operation is performed. An example of such techniques is disclosed in Japanese Patent Prepublication No. 2000-71439.
The above-mentioned prior art has a problem. The gradation of dots is represented by a combination of large, medium, and small dots. At this time, it is difficult to carry out such control that the lightness of each ink color in the print result will be optimized against variation in color component value. As a result, it is impossible to convert image data into dot data so as to obtain a favorable result with respect to all the ink colors. More specific description will be given. In the one-dimensional table according to the prior art, the recording rates of large, medium, and small dots are determined based on the weight of ink. No consideration is given to differences in ink characteristics from one color to another. Therefore, optimum conversion for every color cannot be implemented with one-dimensional tables common to every color. For example, deep inks, such as black ink, are prone to give grainy appearance, and pale inks, such as yellow ink, are less prone to give grainy appearance. Therefore, if both are converted using the same one-dimensional table, it is inevitable to make such an adjustment as to obtain an averagely favorable result for all the colors. Thus, it is impossible to accomplish optimum conversion for every color.